The processing of polymer and elastomer melts in screw extruders is preceded by stirring in a container equipped with suitable agitators or in autoclaves to effect polymerization. With the processing of the material solvents and/or not reacted monomers are reduced to the required residual percentage by degasing and application of heat. The temperature conditions present in the degasing range and also the surface conditions of the melt are of substantial significance.
The purging of the solvent, or the monomer in the melt requires frequently steps which have a damaging effect upon the quality of the melt to be processed; particularly when the end product has to be brought to a very low residual content of such volatile components. As apparent, a minimal percentage of residual components of volatile substances is of great importance for the quality of the end product.
The demand made by industry on the aforereferred to processed materials and especially with respect to highly impact-resistant or tough products is rapidly increasing. To satisfy the demand, it was already necessary, in addition to providing several degasing zones when used in continuously operating screw extruders, to feed in steam or inert gas within the range of the last degasing zone upstream of the discharge zone of the extruder. However, it has been found that such process steps are, in many instances, not sufficient to obtain an acceptable degasing of highly impact-resistant synthetic plastic melts. This is particularly the case when it is important that the process can be carried out at economically acceptable costs when it is demanded that the end product have a residual content of less than 0.5% of volatile components. In any event, when the output of synthetic plastic melts has to be increased the afore-mentioned low percentage of volatile components is difficult to maintain.
U.S. Pat. No. 2,595,455 describes a screw extruder for processing and degasing thermoplastic material. The extruder according to the patent provides within the range of the input or feed-in zone a flow restraining or drag zone in the form of a cylindric bore having within the restraining zone ribs or thread-shaped protrusions. The purpose of such restraining zone is within its working reach an increase of the mixing and shearing effect acting upon the synthetic plastic material to be processed to obtain a loosening of the material. However, it has been found that such increased mixing and shearing forces have no useful effect in the subsequent degasing zone. As a result, the screw extruder arrangement according to the patent does not produce an economically adequate increase of the output capacity of the extruder. It has also been found that the inherently developing additional temperature raise as caused by friction in the pressure zone of the extruder the thermal load acting upon the melt of synthetic plastics material is correspondingly increased.